The antireflective structure that uses an ultrafine periodic structure not larger than the wavelength in the visible light range is known to exhibit superb antireflection performance in a wide range of wavelengths when the ultrafine periodic structure has appropriate pitch and height. An example of the method for forming the ultrafine periodic structure is formation of a film by application of a dispersion of fine particles having a subwavelength diameter.
According to a method for forming an ultrafine periodic structure by forming a pattern with a micromachining device (e.g., an electron beam lithography system, a laser interferometry exposure system, a semiconductor exposure system, or an etching system), the pitch and height can be controlled. It is known that ultrafine periodic structures having good antireflection performance can be produced by this method.
Another example is a method that yields the antireflective effect by allowing a textured structure composed of boehmite, i.e., aluminum oxide monohydrate, to grow on a substrate. According to this method, an aluminum oxide film formed by vacuum deposition or a liquid phase technique (sol-gel technique) is treated with water vapor or immersed in hot water to convert the surface layer into boehmite and form an ultrafine periodic structure, thereby forming an antireflection film (refer to PTL 1).
Although textured structures composed of boehmite can be easily fabricated by such water vapor treatment or immersion in hot water, exposure of substrates to water vapor or hot water is unavoidable. Thus, when the substrate is composed of glass, the glass component may partly elute during immersion in hot water, thereby inhibiting the growth of the boehmite textured structure, or may remain in the textured structure, thereby degrading the antireflection performance. Moreover, since elution of components in the glass readily occurs, some glass component may elute when simply left in a high-temperature, high-humidity environment for a long time, and the performance of the boehmite antireflective film may be deteriorated as a consequence.